(1) Field of the Invention
The present invention relates to a bonded can of the key-opening type and a process for the production thereof. More particularly, the present invention relates to a bonded can of the key-opening type, which is excellent in the key-openability, corrosion resistance and hygienic qualities.
(2) Description of the Prior Art
Key-opening type cans have been widely used as the can for containing therein a livestock product such as corned beef, ham or luncheon meat. As a typical instance of a can of the key-opening type, there can be mentioned a tinplate can in which a seam is formed by soldering, a tongue piece is formed on the outer end portion of the seam and a plurality of scores are formed from the base of the tongue portion along the entire circumference. At the time of opening, the tongue piece is inserted in a winding key, scores are broken by turning the key and a strip-like tinplate between the scores is wound up to effect opening of the can.
This known can of the key-opening type has some serious defects because the seam is formed by soldering. Solder is an alloy of tin and lead and has bad influences on the content. In order to avoid these bad influences, use of wholly tin solder free of lead may be considered. However, the strength of the wholly tin solder is lower than that of the alloy solder, and therefore, when the wholly tin solder is used, it is difficult to form a seam which can resist high-temperature sterilization. At the time of opening, it is necessary to break scores while peeling the seam, and since the cohesive force of the solder is more than 10 times the cohesive force of a resin, a considerably large force is necessary at the start of key-opening of the can. Accordingly, troubles such as breakage of the tongue piece are often caused at the start of key-opening of the can. Furthermore, in case of a tinplate can, a sulfur-containing compound formed by decomposition of meat or the like reacts with metallic tin to cause black coloration, that is, so-called sulfide blackening.
A bonded can composed of tin-free steel (TFS) (such as an electrolytically chromium-coated steel plate) and having a side seam bonded by an adhesive of a thermoplastic resin such as nylon resin has recently been used as a can for carbonated beverages or other beverages instead of the above-mentioned soldered can. It seems strange that the bonded TFS can of this type has not been used as a bonded can of the key-opening type. The reason is considered to be that such troubles as corrosion of the metal in the portion of scores and minute leakages through the scores present in the portion of the seam are not overcome in case of a TFS can blank.